Voice operated systems are typically utilized in communication devices, such as phone devices and computers, as well as in toys. These systems typically comprise such main constructional components as an A/D converter for receiving an input analog voice signal, a vocoder, an operating system, a communication interface associated with an output port, and a voice recognizer (typically implemented as a separate DSP chip).
During a transmission operational mode of the communication device (e.g., mobile phone), the input analog voice signals (e.g., generated by a microphone) are digitized by the converter. In the conventional devices, the digitized voice signals are supplied to the vocoder for compression of the voice samples to reduce the amount of data to be transmitted through the interface unit to another communication device (e.g., mobile phone), and are concurrently supplied to the voice recognizer. The latter receives the digitized voice samples as input, parameterizes the voice signal and matches the parameterized input signal to reference voice signals. The voice recognizer typically either provides the identification of tie matched signal to the operating system, or, if a phone number is associated with the matched signal, provides the associated phone number.
A technique utilizing the application of a voice recognition function to a compressed digitized signal has been developed and disclosed in U.S. Pat. No. 6,003,004 assigned to the assignee of the present application.
It is a well-known problem of voice operated systems that background noise added to speech can degrade the performance of digital voice processors used for speech compression, recognition, authentication, etc. Thus, to improve the quality of voice recognition, it is necessary to reduce the background noise in a speech signal.
Various noise reduction techniques have been developed and disclosed, for example, in the article S. F. Boll “Suppression of Acoustic Noise in Speech Using Spectral Subtraction”, IEEE Transactions in Acoustics, Speech and Signal processing, 1979, V. 27, N. 2, pp. 113-120. According to the known techniques, the noise suppression of the digital signal is typically carried out before the signal is supplied to the vocoder (i.e., prior to signal compression). This approach is therefore computationally intensive and slow. This is a serious drawback when dealing with mobile phones, since the processing requirements of noise suppression and voice recognition pose a severe processing load on the mobile phone and may obstruct its operation. It is known to use an additional DSP chip for noise suppression.